Method of obtaining metallic iron and elemental sulfur from sulfide iron ores



United States Patent() METHOD OF OBTAINING METALLIC IRON AND ELEMENTAL SULFUR FROM SULFIDE IRON ORES Fredrik W. de Jahn, New York, N. Y., assigner of one-half to Alan N. Mann, Scarsdale, N. Y.

Application January 18, 1955, Serial No. 482,446

16 Claims. (Cl. 7S7) This application relates to a method of treating iron sulfide ores (pyrites or pyrrhotite) in such a way vthat not only is the bulk of the sulfur recovered as elementalsulfur but also the iron is recovered as metallic iron in useable form substantially free from sulfur or carbon.

Over the course of years, many suggestions have been made as to methods for obtaining sulfur from pyrites. Except for the practice of distilling oi some of the labile sulfur as elemental sulfur and the usual practice of burning the pyrites to obtain SO2, none of these have gone into commercial operation. My study of the situation indicates that even to such extent as the unused but suggested processes may be operative to obtain sulfur, the percentages of sulfur recovered are so low and the costs of operations are so high that the processes do not pay.

I recognize that among the earlier processes, suggestions have been made that pyrites be treated with steam at relatively high temperatures. In fact, the only practicable method of heating the pyrites to the temperature necessary and without exposing it to carbon-containing gases, is to supply the necessary heat by the steam which takes part in the reaction.

Today there is no process in commercial operation for obtaining elemental sulfur from iron sulfide (FeS') as distinguished from labile sulfur that may be obtained from pyrites (FeSz). My experience indicates that the costs of obtaining sulfur by methods heretofore suggested are so high that the processes are not valuable. In fact, I believe that no such process can have substantial commercial value at present market figures unless it is also possible to obtain the iron in metallic state substantially free from sulfur.

Pyrites will react with steam to give up labile sulfur at temperatures in the order of about 700 C. and the FeS will start to decompose at temperatures somewhat below 900 C. However, at such temperatures the reaction is extremely slow, the amount of steam that has to be employed is large and it is virtually impossible to get all of the sulfur out of the iron. On the other hand, if actual reaction temperatures above 1000 C. (say from 1050" to ll50 C.) are used, the reaction `is very much morerapid and completion of the reaction is much easier to accomplish. However, when one approachesk these temperatures, there is danger that the pyrites willy fuse and stop the reaction. Y

I have overcome these difficulties and have been able to decompose pyrites so that all of the sulfur has been driven out, with the bulk of it in recoverable form, and so that the iron residue is left sulfurfree. I accomplish this result by forming the pyrites into pellets or aggregates which will hold their shape at the high temperatures used ably in an inert atmosphere and such preheating can con er@ .l ICC y 800 C. say from 500 C. to 800 C.

It must be borne in mind that the steam passed into the pyrites mass not only has to raise the temperature of the mass to the reaction temperature but also must supply the necessary heat units for the reaction which is highly endothermic. If the steam only approaches the desired reaction temperature, relatively large amounts of steam have to be employed, whereas if much hotter steam can be used, the quantity can be greatly reduced and this has a marked etect on the economics of the process. By using. a deep bed of pyrites, steam can safely be introduced at a temperature ranging from about l050 C. up to as high as 1300 C. or even somewhat higher if the amount. of steam is at the lower limit. As this steam first strikes the mass, it will contact residues from which virtually all the sulfur has been driven out so that there is little danger of fusion. Then as it progresses through the bed it will progressively contact material containing more and more sulfur. As this occurs, heat units will have to be given up for the reaction of driving the sulfur out of the iron, and this will result in a substantial cooling of the steam. This principal reaction will take place in a zone ranging in temperature from about 950 C. up to about l050 C. and if steam above that temperature is introduced into the bottom of the furnace, it will add to the assurance that all of the sulfur is driven out. After the steam passes beyond the main reaction zone, its function will be to raise the incoming material to the desired reaction temperature and to carry out from the mass the labile sulfur and the sulfur which has been driven from the iron sulde. In this connection it may be noted that if the charge is not preheated and the emerging steam strikes cold pellets of pyrites, the steam will condense, forming moisture which will tend to decompose the pellets and clog the process with condensed sulfur.

As regards the amount of steam employed, I find that I have to use a minimum of about 4 times as much steam by weight as the weight of the sulfur in the charge, as otherwise it is not feasible to carry in the necessary heat units with steam at a permissible temperature. At the top limit it ordinarily will not be commercial to employ more than about l2 times as much steam by weight as the amount of sulfur in the charge. The steam should move through the charge with a substantial velocitycertainly at a minimum of 2 feet per second and preferably at'asubstantially higher speed which may range up to 25 feet per second or even more. It is to be understood that the charge will move slowly but substantially continuously down through the furnace as controlled by an intermittent feed and discharge. For example, the material may remain in the reaction furnace for as much 'as six hours. I do not think that this time is necessary for Vthe bulk of the reaction to take place, but by having the charge exposed to the high-temperature steam after the bulk of the sulfur has been driven out, l am given added assurance that Vthe iron will be sulfurfree.

As a result of this reaction I am able to get sulfur-free iron but even so the iron is in the form of an oxide. In

accordance with my invention it is'contemplated that they state, free from sulfur and it is necessarily free from car- `y bon because no carbon `is introduced in the process.

'In carrying out this process, two specific problems arise,

3 One is the formation of the proper type of pellets or aggregates from the pyrites. The other is the production of carbon-free steam at temperatures such as have been suggested.

As regards the formation of the pellets, I tind that these can most advantageously be formed by mixing finely-ground pyrites (90% through a S25-mesh screen) with a small amount of finely-ground alkaline earth metal sulfate.- The most readily obtained sulfate is calcium sulfate, but if particular circumstances permit, strontium sulfate or barium sulfate may be used.

I have found that 5% of calcium sulfate gives excellent results but this value is not critical. lAs little as 2% may give pellets of adequate strength if they are handled carefully and on the high end one should not use more than It is one of the merits of my process that working at the high temperatures I am able to get a direct evolution of substantial quantities of hydrogen. if the amount of calcium sulfate is unduly increased, the amount of hydrogen produced will be proportionately reduced until a point is reached with about 2 mols of iron sulfide for each mol of calcium sulfate at which no hydrogen will be produced. A good working proportion of calcium sulfate to use is between 4% and 10% based on the weight of the iron sulfide.

The pellets preferably are formed by adding about 10% of water to the mixture of the pyrites and calcium sulfate or other alkaline earth sulfate that may be used. The moistened mass is formed into pellets or aggregates ranging from J{g-inch to l/g-inch in diameter and then thoroughly dried. As an alternative the mixture can be compressed in rotary pelletizing equipment.

I nd that the use of the alkaline earth sulfate as a binder is particularly advantageous. Apparently some reaction takes place between this material and the pyrites which results in a sintering or limited fusion occurring which forms pellets of great strength and these pellets will retain their shape throughout the process. Further, there is an economy in using material containing sulfur, for the sulfur of the binder will be driven out during the process and add to the value of the product. In the case of barium, recovery of barium can also be had by leaching out the barium oxide with hot water to form barium hydroxide. Where the production of such barium hydroxide is a primary objective of the process, special techniques and proportions should be employed, and these are 'the subject of a copending application for patent.

As regards the production of high-temperature steam, a temperature approaching 800 C. is about as high as can be produced commercially in the usual form of superheater where the steam is heated indirectly, that is where the heat has to be passed through an intervening wall. I have found that steam of the desired temperature can be obtained eiciently by superheating steam within ordinary commercial limitations, say from a temperature of 700 C. to 800 C. and then passing this hot steam through a flame of burning hydrogen. The hydrogen preferably is burned with oxygen to avoid the presence of nitrogen. By properly proportioning the amount of hydrogen burned to the amount of steam which is mixed with the ame, an exact control can be had of the desired temperature and in this way I can heat my mass to the desired temperature by the direct heat of the hydrogen flame, using the steam with which the flame is diluted as a medium for carrying the heat units to the mass of pyrites.

As pointed out above, my process includes the production of substantial quantities of hydrogen. This will pass from the reaction furnace with the steam and the sulfur. A part of this sulfur will be in the form of elemental sulfur but much of it will be in the form of mixed H28 and SO2 gases. Under the conditions which I employ and using high temperatures, these will be approximately in the proportion of`2 mols of H28 for each molY of SO2.

Heat units can be recovered from the steam and from these gases by use of a waste heat boiler after which the. steam is condensed. The elemental sulfur will be found in the condensed water in substantially colloidal form and can be recovered by coagulation. The sulfur gases can be caused to react in known manner ordinarily employing a bauxite catalyst. The sulfur-containing gases not removed in this manner can be eliminated by known methods and the purified hydrogen is returned to the process to be used either for burning to heat the steam through the direct flame or it may be used for reduction of the iron oxide.

The reduction step is carried out at a temperature of between about 500 C. and 900 C. and can advantageously be carried out while the pellets still retain from the primary reaction furnace the heat necessary for the reduction. I have found that acting on the pellets in this form, the reduction of the iron is very eicient at about 550 to 650 C. and I also find that if sorne small remnants of sulfide sulfur should still rem-ain in the iron, these will be eliminated by the hydrogen. The reduction with hydrogen does reduce the strength of the pellets somewhat so that they can be readily crushed but they still retain suficient strength to pass through the operation without undue dusting.

After cooling, the reduced pellets consist of virtually pure iron mixed with lime with which will be combined most of the impurities such as silica that may have been included either with the iron ore or with the calcium sulfate. In this connection7 I may point out that it is ordinarily advisable to use enough calcium sulfate so that the lime will form the silica into tricalcium silicate which will be slagged off if the mass is melted. The product in the form of the reduced pellets has a high value, as it can be introduced directly into an electric melting furnace for making carbon-free iron with the lime and silicious impurities going into a slag, or if preferred the pellets may be crushed land subjected to the action of a wet magnetic separator (with substantially oxygen-free water and in a non-oxidizing atmosphere) and in such case virtually pure iron will be obtained.

This invention can readily be understood from the following statement of detailed operation taken in connection with the accompanying drawing which shows a flow sheet of the operation. It is understood that this is lgiven only by way of illustration to show the best method known to me of carrying out my process.

Pyrite ore ground to a fine powder so that passes through a S25-mesh screen (flotation pyrites) is mixed with 5% (based on the weight of the pyrite ore) of calcium sulfate (natural anhydrite) ground to the same fineness. After thorough mixing, 10% of water is added and the material formed into pellets ranging in size from about 1/z-inch to about an inch in diameter. These pellets are thoroughly dried and then introduced into the preheating furnace 10 shown in the accompanying drawing. The air is eliminated from this charge by flushing it with nitrogen. Nitrogen is obtained `as a by-product from the production of oxygen used further on in the process. The material is heated by external heat (burning either oil or gas) to a temperature of about 750 C.

From the preheating furnace the material is passed slowly and approximately continuously into the reaction furnace 12 where a bed about 18 feet deep is formed. Steam at a temperature of about 1100 C. to l200 C. in introduced into the bottom of the reaction furnace as indicated at 14. The solid material is drawn out from the bottom of theV reaction furnace 12 into a cooler 16 and the speed of movement through the furnace 12 is such that it remains in this furnace about 6 hours. This is longer than is required for the reaction, but the presence of the mass of reacted material in the bottom of the furnace where it is subject lto the action of the high temperature steam gives added assurance of having all the sulfur driven out.

The ste-am passing up through the downwardly moving mass of pellets brings them up to a temperature in the main reaction zone of about between 950 and l950 C. Below this reaction zone is an area where the pellets are heated to a somewhat higher temperature to be sure that the last of the sulfur is eliminated.

In the reaction zone, labile sulfur is distilled olf as elemental sulfur and the balance of the sulfur is distilled olf, largely Ias HZS and SO2 in approximately the proportion of 2 mols of H28 for each mol of SO2. There is also a large amount of hydrogen generated which passes off with the steam.

To obtain steam lat the necessary temperature, this is developed in a steam boiler 18 heated in ordinary fashion with oil or gaseous fuel. From the boiler 18 the steam passes into the superheater 20, also heated with fuel in a usual manner, where the temperature is raised to about 790 C. The steam yat this temperature passes into the hydrogen burner 22 where hydrogen is burned with oxygen in the presence of the steam so that the steam is raised by the direct heat of the llame to the temperature of from ll00 C. to l200 C. as previously stated. The amount of steam passing into the furnace 12 is about 12,000 pounds of steam for each 4000 pounds of FeSZ employed (a rati-o of steam to sulfur of 6:-1). For finally heating this amount of steam I use about 19,000 cu. ft. of hydrogen and 9500 cu. ft. of oxygen.

The gases and vapors from the reaction furnace 12 are conducted to a waste heat steam boiler 24 which is used for developing steam to be employed in theY hydrogen generating plant 26. The steam and gases from the reaction furnace are then passed to the condenser 28 where all the labile lsulfur and part of the sulfur from the reaction is `deposited in the condensed water in almost colloidal state. The water carrying this sulfur and the uncondensed gases are conducted to the sulfur recovery plant 30 where the SO2 and H28 pass in mixture through a bauxite catalyst according to known procedure. The sulfur in the water is recovered by coagulation. All of the `sulfur is melted, solidified and taken to storage. The sulfur yield is approximately 90% of the theoretical.

The gas from the sulfur recovery plant consisting primarily of hydrogen is conducted to the hydrogen purication equipment 32 where it is mixed with fresh hydrogen from the hydrogen generating plant 26 and residual hydrogen from the reduction operation. From the purication equipment 32 part of the hydrogen is conducted to the hydrogen burner 22 and another part passes through a heat exchanger 34, then to a hydrogen heater 36, where it is brought up to a temperature of about 700. This hydrogen heater is heated by oil or gaseous fuel in the usual manner.

From the hydrogen heater 36 the hydrogen is introduced into the bottom of the reduction furnace 38 Where it passes upward through the downwardly moving column of pellets discharged from the cooler 16. These pellets are strong and still retain substantially their original shape and size. As they enter the reduction furnace 38 they consist primarily of iron oxide (Fe304) mixed with a small amount of lime (CaO). They are substantially free of sulfur. By the action of the hydrogen on these pellets they are reduced to metallic iron still mixed with a small amount 0f lime. The residual hydrogen then passes out from the furnace 38, through the heat exchanger 34 and back to the hydrogen purification equipment 32. A small proportion of this hydrogen may be bled -out before it is returned to the puriiication equipment and used as fuel in the steam boiler 18. l

The solid product obtained from the reduction furnace 38, while still in the form of pellets, is readily crushable. This product is further cooled to prevent reoxidation and then taken to storage. This product may be either melted in an electric melting furnace giving a yield of pure iron with the lime and other impurities forming a slag on the surface, or as an alternative the product may be treated in a magnetic separator. For this step the material is mixed with water' which has been deaerated and the mixing is done in a neutral atmosphere. The wet mixture of iron -and lime is then passed through a magnetic separator and pure iron powder obtained. This is dried in an inert atmosphere and is available for use as sponge iron having a high commercial value because of its purity and the fact that it is carbon-free.

It is to be noted that in this process hydrogen is generated in the reaction furnace and this amount of hydrogen is a substantial proportion of the entire amount of hydrogen needed in the process. For example, for each 4000 pounds of FeS2 passed into the process an aggregate amount of about 38,000 cu. ft. of hydrogen is used, of which about 10,000 cu. ft. is generated in the reaction furnace 12. This is an important factor in the economics of the operation.

By carrying out my process in this manner I have been able economically to obtain sulfur from the pyrites in elemental form and to obtain the iron from the pyrites in a form of high commercial value. It is free of sulfur and free of carbon.

This application is a continuation-in-part of my earlier application, Serial No. 259,289, filed November 30, 1951, now abandoned.

What I claim is:

1. In the process of decomposing iron sulde ore to obtain elemental sulfur and metallic iron without causing the ore to fuse, the steps of forming nely-ground iron sulde ore into pellets, preheating such pellets to between 500 and 800 C. in an inert atmosphere, progressively feeding the superheated pellets into a reaction furnace to form a bed at least 10 feet thick, progressively heating such pellets from the pre-heat temperature to a temperature of between about 950 C. and 1050 C. by passingsuperheated steam through the mass of pellets in c-ountercurrent, continuing such heating until substantially all the sulfur is driven out of the pellets, leaving a residue consisting primarily of iron oxide, recovering as elemental sulfur the sulfur driven out from thepellet's, cooling the iron oxide residue, drying, and reducing the' iron oxide to metallic iron with a reducing gas at a temperature of between about 500 C. and 900 C.

2. A process as specied in claim 1 in which the superheated steam is introduced into the mass of pellets at a temperature of from 1050 C. to 1300 C.

3. A process as specified in claim 1 in which the weight of steam used is between 4 and 12 times the weight of sulfur in the ore and the steam is passed through the mass at a velocity of at least 2 feet per second.

4. A process as specified in claim 1 in which the sulfur is driven from the ore as elemental sulfur and as a mixture of H2S gas and SO2 gas in approximately the proportion of 2 mols of HZS for each mol of SO2 together with substantial proportions of hydrogen, and which includes the steps of causing the sulfur-containing gases to react to form steam and sulfur and thereafter purifying the residual hydrogen for re-use.

5. A process as specified in claim l which includes the steps of superheating steam to between 700 C. and 800 C. by indirect heat, using such steam to dilute the flame formed by burning hydrogen with oxygen and using such steam to carry the direct heat of the hydrogen llame to the mass of sulfide ore.

6. A process as specified in claim l in which the sulfide ore is formed into pellets by mixing it with from 2% to 20% of finely-ground alkaline earth metal sulfate.

7. A process as specified in claim 6 in which the alkaline earth metal sulfate is calcium sulfate.

8. A process of obtaining metallic iron in useable form from iron sulde ore which comprises `the steps of grinding the ore without causing the ore to fuse, combining it with an amount of a sulfate of an alkaline earth metal sucient to form pellets of substantial strength but in an amount insuicient to prevent the generation 7 of substantial amounts of hydrogen when heated, forming the mixture into pellets,` forming a b ed of such pellets of substantialV depth so.that zones of varying temperature are obtained, slowly moving the solid inaterial downwhile feeding in additional quantities of pellets, passing steam up into the mass of pellets at a temperature of from 1050 C. to 1300 C. whereby the pellets are initially subjected to a temperature below the fusion point but high enough to drive sulfur from the mass and to generate hydrogen, and later after the bulk of the sulfur has been driven oli the pellets are subjected to highertemperatures high enough to drive off the residue of the sulfur without causing the mass to fuse, recovering and separating the sulfur and the hydrogen and treating the pellets from which the sulfur has oxide in the pellets is reduced to metallic iron and any residual sulfide sulfur is removed.

9. A process as specified in claim 8 in which the sulfate of the alkaline earth metal is calcium sulfate and in which thepellets are formed with water. A

10. A process as specified in claim v8l inA which the pellets are preheated to a temperature 01E-between 500 C. and 800 C. in an inert atmosphere before 'being subjected to the action of the steam.

11. A process of obtaining metallic iron in useable form from iron sulfide ore without causing the ore to fuse which comprises the steps of finely grinding the ore and mixing the same with from about 2% to 20% of alkaline earth metal sulfate, forming the powder into pellets, preheating the pellets to a temperature of from about 500 C. to about 800 C. in an inert atmosphere, passing the pellets slowly down through a furnace while passing steam at a temperature of from 1050 C. to 13009 C. upward through the mass of pellets for a sufficient time so that substantially all of the sulfur is driven out of the mass of pellets, cooling the pellets to between 5509 C. and 900 C. and passing a reducing gas through the pellets in the absence of steam whereby the iron oxide present is reduced to metallic iron and residual sulfide sulfate is eliminated.

12. A method as specified in claim 11 which includes the further step of separating the iron from the residue of the alkaline earth metal compound by wet magnetic separation in the substantial absence of free oxygen.

13. A method as specied in claim 12 in which the alkaline earth metal sulfate is calcium sulfate.

14. The method of producing elemental sulfur from iron sulfide without causing the iron sulde to fuse which comprises mixing together calcium sulfate and iron sulfide in the proportion of from 2% to 20% of calicum sulfate based on the weight of the iron sulfide, forming the mixture into pellets and heating such pellets to a temperature of from 950 C. to 1050o C. while passing steam through a mass of such pellets at a velocity of at least two feet per second and at a temperature sufiiciently high to maintain such pellets at the stated temperature and to supply the heat necessary for the reaction, and continuing the reaction until substantially all the sulfur is removed from the iron sulfide.

15. The method as specified in claim 14 in which the mass of pellets is brought up to and maintained at the desired temperature by the direct heat of a hydrogen flame diluted with steam.

16. I n the process of decomposing a major proportion of iron sulfide with a minor proportion of calcium sulfate to obtain sulfur in elemental form without causing the iron sulfide to fuse, the step of bringing the mass up to a temperature of ,about 1000 C. by burning hydrogen, diluting the hydrogen iiame with steam and applying such steam together with the steam generated by Vthe ombustion of the hydrogen direct to the mass to be heated, and continuing such heating until sulfur is distilled off `in elemental form.

References Cited in the tile of this patent UNITED STATES PATENTS 490,847 Grant et al Jan. 31, 1893 680,313 Burrows Aug. 13, 1901 1,083,247 Hall Dec. 30, 1913 1,083,248 Hall Dec. 3.0, 1913 1,104,287 Arnold et al. Iuly 21, 1914 .1,121,606 Birkeland Dec. 22, 1914 

1. IN THE PROCESS OF DECOMPOSING IRON SULFIDE ORE TO OBTAIN ELEMENTAL SULFUR AND METALLIC IRON WITHOUT CAUSING THE ORE TO FUSE, THE STEPS OF FORMING FINELY-GROUND IRON SULFIDE ORE INTO PELLETS, PREHEATING SUCH PELLETS TO BETWEEN 500* AND 800*C. IN AN INERT ATMOSPHERE, PROGRESSIVELY FEEDING THE SUPERHEATED PELLETS INTO A REACTION FURNACE TO FORM A BED AT LEAST 10 FEET THICK, PROGRESSIVELY HEATING SUCH PELLETS FROM THE PRE-HEAT TEMPERATURE TO A TEMPERATURE OF BETWEEN ABOUT 950*C. AND 1050* C. BY PASSING SUPERHEATED STEAM THROUGH THE MASS 